if you want to maintain stocks you are going to have to grow some overnight cultures up,freeze dry them in open vials , and when freeze dried seal up the vials. store at -20 celcius and they should keep indefinately. fialing this you could always maintain viable colonies by replating on to fresh agar every week, however the risk of contamination is great and the is a possibility that after a few months you have isolated a different organism,hope this is of some help.